Light source driving circuit with low operating output voltage

Abstract

A driving circuit for regulating current in a light source using a tracking component. The tracking component detects the voltage difference between an input node in the input stage and an output node in the output stage. The input stage is connected to a current source and includes an input transistor. The output stage is connected to the light source and includes an output transistor. The tracking component generates an output that controls the input and output transistors based on the voltage difference between the input node and the output node so that the voltage at the input node tracks the voltage at the output node. By using the tracking component, the LED driver can achieve accurate current control through one output transistor instead of cascaded transistors, resulting in lower output operating voltage and reduced power dissipation of the LED driver. Further, the tracking component is intermittently operated or shared across different channels to reduce energy consumption of the LED driver.

1. A driving circuit for a light source, comprising:
 an input stage comprising a first transistor connected to a current source or a current sink and a first node between the first transistor and the current source or the current sink; 
 an output stage comprising at least one second transistor for coupling to the light source and at least one second node between the second transistor and the light source; and 
 a tracking component comprising a first input coupled to a first node in the input stage and a second input coupled to a second node in the output stage, the tracking component generating an output signal to control the first transistor and the at least one second transistor so that a voltage level of the first node tracks a voltage level of the second node, wherein the tracking component comprises an amplifier generating an output signal that increases as a voltage difference between the first node and the at least one second node increases, the output signal decreasing as a voltage difference between the first node and the at least one second node decreases; 
 a first switch between the amplifier and a gate of the second transistor, the first switch turned on to receive the output signal from the amplifier in a control mode and turned off in a hold mode; and 
 a second switch between the first transistor and the current sink or the current source, the second switch turned on in the control mode and turned off in the hold mode. 
 
     
     
       2. The driving circuit of  claim 1 , wherein an output of the amplifier is coupled to the gate of the first transistor and the gate of the second transistor. 
     
     
       3. The driving circuit of  claim 1 , further comprising a third switch between the gate of the second transistor and ground, the third switch turned on to disable the light source coupled to the output stage. 
     
     
       4. A driving circuit for a plurality of light sources, comprising:
 an input stage comprising a first transistor connected to a current source or a current sink and a first node between the first transistor and the current source or the current sink; 
 an output stage comprising a plurality of channels, each channel comprising a second transistor for coupling to a light source and a second node between the second transistor and the light source, each channel connected to an amplifier in a sequential manner to control input current in the light source coupled to the channel; and 
 a tracking component comprising a first input coupled to a first node in the input stage and a second input coupled to a second node in the output stage, the tracking component generating an output signal to control the first transistor and the at least one second transistor so that a voltage level of the first node tracks a voltage level of the second node, wherein the tracking component comprises the amplifier generating an output signal that increases as a voltage difference between the first node and the at least one second node increases, the output signal decreasing as a voltage difference between the first node and the at least one second node decreases. 
 
     
     
       5. The driving circuit of  claim 4 , wherein each channel comprises a first switch and a second switch, the first switch placed between the amplifier and a gate of the second transistor of the channel, the first switch turned on to receive the output signal from the amplifier in a control mode and turned off in a hold mode, the second switch placed between the first transistor and the current sink or the current source, the second switch turned on in the control mode and turned off in the hold mode. 
     
     
       6. The driving circuit of  claim 1 , wherein the light source comprises a light emitting diode. 
     
     
       7. A method of controlling an output current of a light source, comprising:
 receiving a first voltage signal from a first node between a first transistor and a current source or a current sink in an input stage; 
 receiving a second voltage signal from a second node between a second transistor and the light source in an output stage; and 
 adjusting first gate voltage of the first transistor and second gate voltage of the second transistor so that a voltage level of the first node tracks a voltage level of the second node; 
 generating an output signal at an amplifier that increases as a voltage difference between the first node and the second node increases, and decreases as a voltage difference between the first node and the second node decreases, the output signal provided to a first gate of the first transistor and a second gate of the second transistor; 
 turning on a first switch between the amplifier and a gate of the second transistor to receive the output signal from the amplifier in a control mode; 
 turning off the first switch in a hold mode to disconnect an output of the amplifier from the gate of the second transistor; 
 turning on a second switch between the first transistor and the current sink or the current source in the control mode; and 
 turning off the second switch in the hold mode. 
 
     
     
       8. The method of  claim 7 , further comprising:
 turning on a third switch between the second gate of the second transistor and ground to disable the light source coupled to the output stage. 
 
     
     
       9. A method of controlling an output current of a plurality of light sources, comprising:
 receiving a first voltage signal from a first node between a first transistor and a current source or a current sink in an input stage; 
 receiving a second voltage signal from a second node between a second transistor and the light source in an output stage; 
 adjusting first gate voltage of the first transistor and second gate voltage of the second transistor so that a voltage level of the first node tracks a voltage level of the second node; 
 coupling a first channel in the output stage to the amplifier responsive to receiving a first switching signal, the first channel coupled to a first light source; and 
 coupling a second channel in the output stage to the amplifier responsive to receiving a second switching signal, the second channel coupled to a second light source, the first switching signal and the second switching signal not overlapping in time. 
 
     
     
       10. The method of  claim 9 , wherein the first switching signal and the second switching are received in a sequential manner. 
     
     
       11. The method of  claim 7 , wherein the light source comprises a light emitting diode.